Seismic Imaging of the Mariana Subduction Zone: An Overview
Abstract
We present an overview of seismic imaging results from the Mariana subduction zone, focusing on the 2003-2004 and 2012-2013 passive ocean bottom seismograph surveys and the 2012 Langseth active source survey. The incoming Pacific plate is hydrated to depths of 30-40 km below seafloor at the trench axis, as indicated by shear wave velocities imaged by ambient noise and earthquake Rayleigh waves (Cai et al., 2018). Strong anisotropy is observed, with fast axes parallel to the trench, consistent with shape preferred orientation caused by slow velocities along faults. Active-source data reveal a P velocity structure of the crust and uppermost mantle that is consistent with the passive S-wave structure (Eimer et al., in prep). Earthquake studies using the same OBS dataset show that seismic activity in the incoming plate increases approaching the trench axes, consistent with increased seafloor fault offsets, and extensional earthquakes occur to depths of 30-35 km below seafloor (Eimer et al., in prep). These results indicate that the incoming plate mantle experiences approximately 20% serpentinization to depths of 30-40 km. The outer forearc shows slow seismic velocities in both the passive and active source results, which we interpret as more than 50% serpentinization in the mantle portion and extensive deformation and alteration of the crust in the outermost forearc and underneath a serpentine mud volcano. Body wave tomographic images show separate slow seismic velocities centered at depths of 60-70 km beneath the volcanic arc and 20-30 km beneath the backarc spreading center, consistent with depth estimates for melt equilibration from thermobarometry (Barklage et al., 2015). A major conclusion is that much more water subducts at the Mariana Trench than was previously estimated. Free water in cracks and pore spaces is expelled and flushes the outer forearc, extensively serpentinizing the mantle and weakening the crust. The amount of water carried to sub-arc depths is about 4 times greater than previous estimates, suggesting that estimates of water expelled at the arc and backarc also must be increased.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V54B..07W
- Keywords:
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- 3613 Subduction zone processes;
- MINERALOGY AND PETROLOGY;
- 8170 Subduction zone processes;
- TECTONOPHYSICS;
- 8185 Volcanic arcs;
- TECTONOPHYSICS;
- 8413 Subduction zone processes;
- VOLCANOLOGY